The present invention relates to an automotive engine cooling system having a thermostat, and more particularly to a cooling system for controlling the circulation of coolant when the automotive engine is cold.
As shown in FIG. 7, a conventional cooling system for an automotive engine comprises a first coolant passage 6 disposed between an upper outlet 5 of water jackets 4 and an upper inlet 12 of a radiator 11, and a second coolant passage 14 provided between a lower outlet 13 of the radiator 11 and a lower inlet 10 of the water jackets 4, including a thermostat cap 16, a thermostat housing 8 and a water pump 9. A bypass passage 7 is provided between a junction J of the first passage 6 and the housing 8 so as to communicate the first passage 6 with the second passage 14 without passing the radiator 11. A thermostat 1 is secured to the housing 8 by the thermostat cap 16. The thermostat 1 has a main valve 3 and a bypass valve 2. In FIG. 7, the reference A' designates a measuring point for measuring the temperature of the coolant in the housing 8, and B' designates a measuring point provided in the second passage 14 adjacent to the cap 16 and upstream of the thermostat 1 for measuring the temperature of the coolant in the second passage 14. C designates a measuring point for measuring the flow rate of the coolant in the second passage 14.
During the engine is warmed up, the main valve 3 of the thermostat 1 is closed, while the bypass valve 2 integrated with the main valve 3 is fully opened. Thus, the coolant drawn from the outlet 5 of the water jackets 4 does not pass through the radiator 11. The coolant is circulated by the water pump 9 through the junction J of the first passage 6, bypass passage 7, housing 8, and inlet 10 of the water jackets 4 as indicated by arrows. Thus, the temperature of the coolant in the housing 8 quickly rises.
However, since the coolant in the radiator 11 and the thermostat cap 16 is not circulated, the temperature rising rate of the coolant therein is slow. Therefore, as shown in a record of FIG. 8, even if temperature A at the point A' becomes 85.degree. C. which is an opening temperature of the main valve 3, temperature B at the point B' is 56.degree. C. There is a difference of 29.degree. C. between the temperatures A and B.
When the main valve 3 of the thermostat 1 opens, the coolant of a low temperature is drawn from the lower outlet 13 of the radiator 11 and fed to the thermostat housing 8 through the second passage 14. Consequently, the temperature B of the coolant at the point B' is further lowered by 20.degree. C. As a result, the difference between the temperature B of the coolant in the passage 14 and the temperature A of the coolant in the housing 8 becomes 49.degree. C.
Since the heat sensitivity of the thermostat 1 is low, the response of the thermostat delays with respect to the change of the coolant temperature. Therefore, the main valve 3 opens after the temperature has become higher than a predetermined opening temperature. Similarly, the main valve 3 closes after the coolant temperature has considerably decreased lower than a predetermined closing temperature. Namely, there is a large heat overshoot in control of the coolant temperature, which causes the main valve to be repeatedly opened and closed. When the main valve 3 closes, a surge pressure occurs at the upstream of the main valve. These variations of temperature and pressure are repeated, and gradually reduced, and faded away as the valve lift of the main valve 3 increases. Such a variation of the temperature is definitely shown in FIG. 8.
Such a heat overshoot causes cracks of the cylinder block and cylinder head, and the surge pressure causes breakdown of the thermostat 1 and the radiator 11. The fluctuation of the hydraulic pressure causes overload on the water pump 9, which will shorten the life of the pump. Furthermore, an abnormally low temperature of the coolant at the opening of the main valve affects combustion condition in cylinders, which causes deteriorating the emission control and increasing fuel consumption of the engine.
These troubles are mainly caused by a large difference between the coolant temperatures A and B. Therefore, it is necessary to reduce the temperature difference for solving the problems.